Image scanning apparatus using a contact image sensor, hereinafter simply referred to as a “scanner”, is commonly used today to electronically capture images from printed documents by scanning the document while the document is held in contact with the scanning surface of the sensor. The scanning surface is typically a glass plate. To capture images from the document with high precision requires pressing the document against the glass plate with appropriate force.
Conventional scanners, which have a contact image sensor, typically transport the document over the scanning surface using, for example a spring to press the document against the scanning surface. In some scanners the pressure which pushes the document against the scanning surface, is adjustable as taught, for example, in JP-A-6-211132, FIG. 1, paragraphs [0016] to [0018]).
FIG. 15 illustrates a conventional pressure generating mechanism for the scanner taught in JP-A-6-211132 having a platen roller 102, which is controllably mounted so as to press a document against the scanning surface of the sensor 101. By turning a pressure adjustment gear set 105, the pressure control lever 104 pivots on a stationary shaft at the left distal end of the lever 104 which, in turn, adjusts the degree of compression of the pressure control spring 103 corresponding to the manual adjustment of the gear set 105. The sensor 101 is positioned in a horizontal plane in FIG. 15 and can move only in the vertical direction.
The pressing force F in FIG. 15 between the sensor and document when disposed between the platen roller and scanning surface of the sensor can be calculated from the following equationF=kx−mgwhere k is the spring constant, x is spring compression, m is the self-weight of the sensor, and g is the acceleration of gravity. As this equation shows, force F can be adjusted as desired by turning the pressure adjustment gear 105, thereby changing the compression x of the pressure control spring 103.
The configuration as shown in FIG. 15 makes it easy to manually adjust the force F with little required torque because of the worm gear used in the pressure adjustment gear set 105. Furthermore, once the pressure is adjusted and set as desired, the pressure adjustment gear set 105 reliably holds the setting. Additional means for holding the pressure setting is therefore unnecessary.
Document-feeding type scanners using a conventional contact image sensor are designed to transport and scan documents having a thickness in a specific range. As a result, when particularly thick documents, which exceed this thickness range need to be scanned such as a driver's license and card stock (referred to below as “thick originals”), the pressure on the thick original may be insufficient to press it against the platen and have little resistance to the tendency of the document to bend. Accordingly, thick originals are not held against the scanning surface of the sensor with sufficient force to prevent scanning errors from occurring.
On the other hand, if the pressure is set for thick originals and a document which is thinner than the set thickness range (a “thin original”) is then scanned, the excessive pressure could wrinkle the original and cause document feed errors, e.g. paper jams.
Furthermore, the surface of the original can be easily marred if the original is thin and heat-sensitive or is a pressure-sensitive paper. In such cases it is not desirable to apply more pressure than needed.
Moreover, the paper feed load may change at the moment the roller pair grabs the edge of a thick original and at the moment the rollers release the edge. This will disturb the paper feed pitch and can adversely affect image scanning. This fluctuation in the feed load can be reduced by using larger rollers, but this increases the size of the drive device.
The conventional scanner as taught in JP-A-6-211132 addresses some of the above problems by providing a manually adjustable means using the pressure adjustment gear to change the compression setting (initial compression) of the pressure control spring 103 which, in turn, varies the force F according to the thickness of the original document.
The conventional scanner shown in FIG. 15, however, adjusts the force on the original and platen using the pressure generating mechanism by changing the compression of only one spring 103, and can therefore only use one spring constant to set the applied pressure. The problem with this scanner, therefore, is that the range in which the applied pressure can be adjusted is necessarily limited, and thin originals and thick originals of greatly different thickness cannot be pressed against the sensor with pressure appropriate for the specific original.
Pressure adjustment in the conventional pressure generating mechanism shown in FIG. 15 requires the operator to manually manipulate a pressure adjustment gear to adjust the pressure each time the thickness of the original changes. Thus setting the pressure correctly is complicated.